|Quadrangle map, 1:250,000-scale||LC|
|Quadrangle map, 1:63,360-scale||C-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||Located at the headwaters of a tributary approximately 5 miles to the south of the Chilikadrotna River, approximately 30 miles northwest of Lake Clark. The prospect is along a crescent shaped ridge immediately south of hill 3213 in the northeast 1/4 of section 16, T. 5 N., R. 33 W. of the Seward Meridian. Location is accurate to within 200 meters.|
The area between the Koksetna and Chilikadrotna Rivers is underlain by undivided Kuskokwim Group sedimentary rocks and the Koksetna River sequence of Wallace and others (1989) that have been intruded by Cretaceous to Tertiary quartz monzonite plutons and rhyolite to dacite plugs, sills, dikes and flows (Wilson and others, 2006). The Kosmic prospect lies within the Kuskokwim Group.
Quartz monzonite porphyry occurs as a small stock approximately one kilometer in diameter and is the dominant intrusive rock at Kosmic. Quartz monzonite porphyry is characterized by abundant phenocrysts of smoky quartz 3 to 5 millimeters in diameter and plagioclase with less abundant potassium feldspar phenocrysts (Scott and Ellis, 1982).
Rhyolite porphyry occurs as dikes emanating outward from the quartz monzonite porphyry. Intrusive contacts between rhyolite and quartz monzonite porphyry were not observed and at one location quartz monzonite porphyry appears to grade into rhyolite porphyry. Moderate fracturing and contact metamorphism extends outward roughly 100 meters from the intrusive-sedimentary rock contact and dies out rapidly between 100 and 200 meters from the contact (Ellis and others, 1985).
The strongest alteration occurs in rhyolite porphyry although the majority of the rocks at Kosmic are unaltered. Alteration of rhyolite porphyry is primarily sericitic and includes localized patches of silicified, tourmalinized rock where black tourmaline occurs in fractures and as rosettes throughout the rock (Ellis and others, 1985).
The rhyolite and quartz monzonite are locally cross-cut by veins; however, veining is more common in the hornfelsed sedimentary rocks. Four types of veins were observed. Type 1 veins occur in the hornfels and are 0.5 centimeter wide fractures filled with quartz, minor muscovite, and rare traces of arsenopyrite. Type 1 veins have no alteration envelopes. Some of these veins carry anomalous silver and gold. Type 2 veins are 1.0 to 5.0 millimeters wide fractures in quartz monzonite and rhyolite porphyry. These fractures are filled with quartz and minor arsenopyrite with scorodite. Muscovite envelopes extend outward approximately 1.0 to 5.0 centimeters from these veins. Grab samples from these veins carry anomalous silver and tin. Type 3 veins occur in hornfels near intrusive contacts as coarsely crystalline veins and pods up to 10 centimeters in diameter. These pegmatic veins contain glassy quartz, muscovite, biotite, and potassium feldspar. Dark green or black tourmaline crystals and very rare cassiterite occur in these veins. Type 3 veins are probably late pegmatic phases of quartz monzonite porphyry. Type 4 veins occur in hornfels as sinuous veins and pods of milky white metamorphic quartz with clots of biotite and muscovite. These veins are common in the area and are produced by contact metamorphism. At the Kosmic prospect type 4 veins frequently contain coarse splays of pale green tourmaline (Ellis and others, 1985).In addition to vein-type mineralization described above, three patches (approximately 10 meters in diameter each) of mineralized rhyolite porphyry were mapped on the east flank of the Kosmic prospect. The mineralization occurs in a narrow (20 meter) east-west rhyolite porphyry dike that extends onto the adjacent ridge. The mineralized rock contains 1 to 3 percent disseminated arsenopyrite and includes minor amounts of chalcopyrite, covellite, and molybdenite(?). Covellite coats chalcopyrite and arsenopyrite and has copper values to 695 ppm (Ellis and others, 1985).
|Geologic map unit||(, )|
|Mineral deposit model||Polymetallic veins? (Cox and Singer, 1986; model 22c).|
|Mineral deposit model number||22c?|
|Age of mineralization||Mineralization is probably related to 60.5 to 61.6 Ma intrusions dated in the area (Eakins and others, 1978).|
|Alteration of deposit||The strongest alteration occurs in rhyolite porphyry although the majority of the rocks at Kosmic are unaltered. Alteration of rhyolite porphyry is primarily sericitic and includes localized patches of silicified, tourmalinized rock where black tourmaline occurs in fractures and as rosettes throughout the rock (Ellis and others, 1985).|
|Workings or exploration||
Anaconda Minerals Company (Anaconda) discovered the Kosmic prospect during a regional exploration in 1982 (Scott and Ellis, 1982). In 1983 Anaconda mapped and sampled Kosmic identifying four types of veins, three of which were mineralized. Type 1 veins in hornfels sometimes carried elevated silver up to 27 parts per million (ppm) and gold up to 4430 parts per billion (ppb). Grab samples from type 2 veins in the quartz monzonite and rhyolite porphyry carried elevated silver up to 17 ppm and tin up to 1450 ppm. Type 3 veins contained rare cassiterite, identified in hand specimen and in thin section. Eleven grab samples from mineralized rhyolite porphyry with 1 to 3 percent disseminated arsenopyrite and minor chalcopyrite, covellite and molybdenite (?) contained anomalous silver (1.3 to 30 ppm), arsenic (over 1000 ppm), and fluorine (over 5000 ppm). Five out of 22 samples had anomalous tungston values from 100 to 475 ppm. Three samples of greisenized rhyolite contained 395, 440, and 565 ppm tin (Ellis and others, 1985).An induced polarization (IP) and resistivity anomaly at the north end of Kosmic was discovered in 2007 when Andover Ventures Inc. completed a 2.7 kilometer long geophysical survey along the crescent ridge. Rock and soil sampling along the IP line detected anomalous arsenic, copper, and zinc (Ellis and Hoffman, 2008).
|Indication of production||None|
|Reporters||W.T. Ellis (Alaska Earth Sciences)|
|Last report date||3/15/2016|